Chemical adhesives, such as those based on epoxy or free-radically initiated compositions, may include two or more components that react together and cure when mixed. Such adhesive compositions are widely used for a variety of applications, and are dispensed in a variety of different, convenient motifs. For example, U.S. Pat. No. 5,730,557 discloses a mortar mixture capsule unit for chemical attachment of anchors in boreholes. The capsule houses a filler material, and a chemical binder system contained in smaller capsules within the filler material. The cartridge is inserted into a borehole, and a driving tool is used to insert an anchor. Pressure exerted on the cartridge by the inserting anchor ruptures both the housing capsule and the smaller capsules contained within it, causing the chemical binder system to mix with the filler, thereby curing the binder system/filler mixture and securing the anchor within the borehole. A similar adhesive is sold by Hilti AG under the trade name “HVU”.
U.S. Pat. No. 5,731,366 discloses a chemical plugging compound based on a free-radically polymerizable resin and a free-radical initiator provided in a separate (non-mixing) state, such as encapsulated in glass, gelatin or cellulose capsules.
Such adhesives have certain disadvantages, primarily the lack of control of metering. Because one or both components are fluid, and must be encapsulated in a package prior to use, it is at the least inconvenient to vary the size of a capsule or the amount of adhesive in the capsule, and hence, for applications such as securing anchors, the boreholes can end up receiving the same amount of pre-packaged adhesive irrespective of borehole size. As a result of inadequate metering, excess adhesive may flow or spill prior to full cure, particularly where the borehole is upside down and vertical as in a ceiling or roof, and cartridges can fall from overhead boreholes. Furthermore, such encapsulation techniques do not assure that the smaller capsules used to contain the binder and/or initiator will remain evenly dispersed, which can lead to uneven or inadequate reinforcement of the anchor.
Alternatively, adhesives in which the binder and curing agent are dispensed from separate cartridges may be used, where the separate cartridges are simultaneously injected into a borehole using a two-barrel epoxy gun which brings the two parts together at the point of injection, whereupon they react upon entering the borehole. Such epoxy guns are commonly designed to inject the contents of each tube at the same rate, with the same volume of binder and curing agent being dispensed simultaneously. However, for such a system to function properly and avoid the problem of variations in metering, the adhesive composition must provide optimum performance when combined in a 1:1 ratio by volume, which is a typical design configuration of epoxy guns.